Printing devices such as laser printers, digital photocopiers and fax machines use a laser beam to write an image on a photosensitive surface. The surface moves and the laser beam scans an image by sweeping in a direction perpendicular to that of the moving surface. This scanning action is similar to the movement of an electron beam across a television tube or other cathode ray tube (CRT). However, unlike a CRT, one dimension of the image, call it the Y dimension, is controlled by the movement of the surface, while the other dimension, call it the X dimension, is controlled by the scanning action of the laser.
Laser imaging devices implement the scanning action by reflecting a laser beam off a rotating reflector, often referred to as a rotating mirror. The rotating mirror is a reflector typically having two or more faces called facets that reflect the laser beam. Mirrors with two, four, six or eight facets are not uncommon.
In a laser imaging system having a mirror with a plurality of facets, the quality of an image is affected by several factors related to the design and manufacture of the facets. An ideal system operates within the constraints listed below.                (1) In the case of an even-sided polygonal mirror, facets on opposite sides of the mirror are parallel to one another.        (2) The mirror has minimal run-out relative to its rotational center.        (3) The angles formed by the facets of the polygon are precise.        (4) The motor and bearing system run true, without wobble.        
A system that fails to meet these constraints can produce objectionable artifacts in a printed image. These artifacts are due to scan lines of different lengths.
In a system using a multi-facet mirror, successive facets of the mirror produce successive scan lines of the image. Thus, a specific facet of the rotating mirror produces specific scan lines. For example, a four-faceted mirror will produce scan lines as shown in Table 1, below.
TABLE 1Four-Faceted MirrorFacet NumberScan Line Number112233441526374819210311412
Imperfections in the mirror facets can cause scan lines to be of different lengths. For example, all the scan lines written by a facet can be of one length, while those written by another facet are a different length. Presently, scanners can produce scan lines with 300 or 600 dots per inch (DPI) that vary less than one dot per line. Nonetheless, even a minor difference in the length of a scan line can cause a periodic distortion in an image.
The beginning of each scan line is electronically synchronized to a starting margin of an image. The synchronizing signal is conventionally known as a “beam detect” (BD). A variation in the scan line accumulates over the length of the scan line and typically reaches its maximum at the end of the scan line.
An observer will usually not notice any variation in a single line. However, a periodic pattern produced by the variation in the scan line may interfere with a pattern of gray scale or halftones in an image, thus creating a moiré pattern. A moiré pattern typically appears as a periodic series of lines superimposed over the image. Even though differences in the lengths of the scan lines are less than one dot wide, the human vision system is very sensitive to moiré patterns caused by errors of less than the width of one dot. For example, a human can detect a moiré pattern caused by a ¼ dot error in an image produced by a 600 DPI printer. As laser printers are called upon to print images approaching photograph quality, gray scale and halftone patterns are used more frequently, and the resulting images are more susceptible to moiré.
Methods exist that compensate for scanner imperfections by lengthening or shortening the lines produced by the facets until each line is the same length. A system applying such a method requires knowledge of:                (1) the amount of facet-to-facet imperfection, and        (2) which facet of the mirror is reflecting the laser beam.        
Knowledge of the amount of facet-to-facet imperfection is used to determine how much compensation is required for a particular facet. A facet error can be characterized in terms of the time it takes to sweep a beam across a predetermined length. For example, a 600 DPI printer that prints a line across an eight-inch page prints 4800 dots.4800 dots=600 dots/inch×8 inches
A particular model of printer may print a dot in 50 nanoseconds (ns). Thus an eight-inch line would be printed in 240 microseconds (μs).240 μs=4800 dots×50 ns/dot
If the nominal scan line is 240 μs long, then a scanner imperfection that causes a scan line length of 240.050 μs corresponds to a length of one extra dot. A scan line length of 239.950 corresponds to a line that is one dot shorter than the nominal line. As humans can detect moiré patterns caused by a ¼ dot error, a facet-to-facet deviation of 12.5 ns can result in a noticeable imaging artifact.
Knowledge of which facet of the mirror is reflecting the laser beam is necessary so that an appropriate compensation can be applied when a particular facet is producing a scan line. The scan line lengths are corrected on a facet-by-facet basis so that all the resulting printed lines on the page are the same lengths.
One technique for characterizing a scanner involves the use of a test fixture to measure and record the scan line length of each facet. This information is either physically written onto the scanner, or stored into an electronic memory that is included with the scanner. The information is subsequently recalled during a line length correction procedure. Because the measurement is made external to the scanner system, this technique requires additional manufacturing steps for the characterization process, and further requires a step for a transfer of the characterization information from the scanner to a compensation circuit. Both steps add to the cost of the scanner.
Another technique for characterizing a scanner involves the use of a high-speed counter to accurately measure the scan line length. Dots are printed at a rate based on a dot frequency. For example, if a dot is printed in 50 ns, its effective dot frequency is 1/50 ns or 20 MHz. A counter that measures a line in a scanning system is typically clocked at least eight times faster than the dot frequency in order to provide ⅛ dot resolution. In this example, the clock frequency would be at least 160 MHz.160 MHz=8×20 MHz
A higher clock frequency would provide an even more accurate measurement of the scanner error. For example, a 1 GHz clock would provide a timing resolution of 1 ns, which corresponds to 1/50 of a dot. Disadvantageously, the higher the clock speed at which a circuit operates, the higher is the cost of electronic circuitry and the greater is the potential for radiated noise.
An existing technique for identifying which facet of a mirror is reflecting the laser beam is to tag one facet so that the facet can be detected by a sensor. For example, the tag might be a physical mark that is optically sensed. Assume that facet #1 is tagged. The sensor will detect facet #1, and thereafter a beam detect circuit counts subsequent facets until the mirror makes a full rotation bringing facet #1 into printing position again. Disadvantageously, this technique requires a means for tagging a facet, a sensor for detecting the tag, and wiring to communicate the facet information to the compensation circuitry.
Accordingly, there is a need for a method of characterizing a scanner facet error that is performed within a scanner system without requiring high-speed electronic circuitry.
There is also a need for a method of identifying which facet is reflecting a laser beam without requiring a sensor for detecting a particular facet.